Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.

 

The man has violated the law of the land as well as the sacred law of Islam by building a mosque in this way. As such, Muslims should not go and pray in it; rather they should continue praying in the city mosque in the proximity.

 

If what you allege is true then this `masjid` is not built on a foundation of piety; rather it resembles the Masjid al-dirar (fake or evil mosque built by the hypocrites in Madinah without the sanction of the Prophet (peace be upon him):

 

“And [there are hypocrites] who have established a [separate] house of worship in order to create mischief, and to promote apostasy and disunity among the believers, and to provide an outpost for all who from the outset have been warring against God and His Apostle. [142] And they will surely swear [to you, O believers], “We had but the best of intentions!” – the while God [Himself] bears witness that they are lying.” (At-tawbah 9: 107)

 

In light of these, the brother is advised to pull down this `masjid` since it violates both the secular and religious laws.

 

Almighty Allah knows best.